A.c. electrical properties of vacuum deposited Au-SiOx-Au sandwich structures prior to electroforming

Abstract

Vacuum-evaporated silicon oxide (SiOx films of thickness 150 nm were fabricated in the form of sandwich structures using Au electrodes. It is well known that such samples undergo an electroforming process subsequent to the application of a forming voltage after which voltage-controlled differential negative resistance (VCNR) is observed. The a.c. electrical properties of such structures were studied prior to electroforming in the frequency range 100 Hz–20 kHz and for temperatures in the range 78–523 K. The a.c. conductivity σ(ω) showed a power-law dependence on angular frequency ω, with exponent s in the range 0.5 ⩽ s ⩽ 1.0 below 1 kHz and s ⩾ 1.0 above 10 kHz. The low frequency results obtained at temperatures below approximately 323 K. showed a good agreement with the model of Elliott for amorphous structures, yielding a typical low-temperature activation energy of 1.4 meV, strongly indicating a hopping mechanism. Sample capacitance showed a moderate decrease with frequency and an increase with temperature, eventually becoming constant at temperatures below 300 K for all frequencies. The loss tangent tan δ showed a minimum in its frequency dependence at low temperatures, which shifted to higher frequency with increasing temperatures. For temperatures above 423 K, the minimum in tan δ was not observed below 20 kHz. The results are discussed in terms of existing models, a good correlation being obtained with the model of Goswami and Goswami which assumes a thermal activation process and the existence of ohmic contacts to the SiOx.